docs: update PLAN REPLAYER object storage behavior

[zeminzhou]

Summary

Backport the PLAN REPLAYER documentation update to release-8.5.

• document that PLAN REPLAYER can store generated files in the storage configured by tidb_cloud_storage_uri
• clarify that PLAN REPLAYER DUMP returns either a file token or a presigned URL depending on the backend capability
• document the corresponding download flow and presigned URL validity
• clarify that PLAN REPLAYER CAPTURE files can also be stored in external storage while the token column still records the file token
• update related system variable descriptions in system-variables.md

Reference

• Cherry-pick of commit b9455277b9c3215b97624203b287adf3de8e3184
• Related mainline PR: docs: update PLAN REPLAYER object storage behavior #22548
• Based on planner: introduce external storage for plan replayer tidb#65477, planner: store plan replayer files to obj store tidb#65904, planner: add presigned url to download files for replayer tidb#66084

Testing

• Not run (documentation only)

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the documentation for the PLAN REPLAYER feature to reflect its enhanced integration with external object storage. The changes clarify how generated plan files are stored, retrieved, and managed when tidb_cloud_storage_uri is configured, providing users with comprehensive guidance on utilizing cloud storage for plan replayer outputs.

Highlights

• External Storage Support for PLAN REPLAYER: Documented PLAN REPLAYER's support for storing generated files in external storage configured by tidb_cloud_storage_uri.
• Flexible Output for PLAN REPLAYER DUMP: Clarified that PLAN REPLAYER DUMP can return either a file token or a presigned URL, depending on the storage backend's capabilities.
• Enhanced Download Instructions: Provided detailed instructions for downloading PLAN REPLAYER DUMP files, including direct usage of presigned URLs.
• PLAN REPLAYER CAPTURE Storage: Updated documentation to state that PLAN REPLAYER CAPTURE files can also be stored externally, while the token column continues to record the file token.
• System Variable Updates: Revised system variable descriptions in system-variables.md to accurately reflect PLAN REPLAYER's new storage behaviors.

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Changelog

• sql-plan-replayer.md
  • Added a section explaining that PLAN REPLAYER can store generated ZIP files in external storage if tidb_cloud_storage_uri is set.
  • Updated the description of PLAN REPLAYER DUMP's return value to specify it can be a presigned URL or a file token.
  • Included an example demonstrating a presigned URL as the output of PLAN REPLAYER DUMP.
  • Provided curl command examples for downloading files using presigned URLs.
  • Clarified the download process for PLAN REPLAYER CAPTURE files when external storage is used.
  • Revised notes on file retention periods to cover both local and external storage.
• system-variables.md
  • Expanded the description of tidb_cloud_storage_uri to explicitly mention its use by PLAN REPLAYER for storing generated files.
  • Updated the description of tidb_last_plan_replayer_token to indicate that its result can be either a file token or a presigned URL.

Activity

• This pull request is a backport of a documentation update to the release-8.5 branch.
• The changes are purely documentation-related, so no testing was performed.
• References to related mainline PRs (docs: update PLAN REPLAYER object storage behavior #22548, #65477, #65904, #66084) and the original commit (b9455277b9c3215b97624203b287adf3de8e3184) are provided.

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[gemini-code-assist]

Code Review

This pull request updates the PLAN REPLAYER documentation to reflect its new capabilities with external object storage. The changes are well-written and significantly improve the accuracy and clarity of the documentation. I've included a few minor suggestions to align the text more closely with the repository's style guide, primarily focusing on using active voice and more direct phrasing for better readability.

_{Note: Security Review has been skipped due to the limited scope of the PR.}

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[zhaoshangzi]

If tidb_cloud_storage_uri setted , what are the contents of column 'token'?

[zhaoshangzi]

Will TiDB delete the result file on external storage after one hour ?